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Looking for Life Beyond Earth: The Search for Habitable Exoplanets

The search for potentially-habitable planets has heated up in recent years, and it's only going to get hotter as the decade unfolds!

What would it be like to discover intelligent life elsewhere in the Universe? Chances are, we've all thought about it at one time or another. And for generations, the greatest scientific minds in the world have speculated about the odds of finding it, and what forms it might take.

While we have barely scratched the surface, we are at a pivotal time in our search for life elsewhere in the Universe. This is largely due to the way modern telescopes have allowed us to discover thousands of extrasolar planets (or just exoplanets).

As the number of confirmed exoplanets has grown, the focus has been slowly shifting from discovery to characterization. In other words, we've found many distant worlds, now we're attempting to determine which of them might be able to actually support life.

In the coming years, we stand to many more planets and learn a great deal more about the ones we already know about. But first, a few things need to be clarified, not the least of which is terminology.

Looking for Life Beyond Earth: The Search for Habitable Exoplanets
Artist's impression of Proxima b, Source: NASA

What are extrasolar planets?

The term extrasolar planet (exoplanet for short) refers to planets that are beyond our Solar System. For centuries, astronomers have speculated about the existence of planets around other stars. However, it was not until the late 1980s and early 1990s that the first confirmed discoveries were made.

The first occurred in 1988 when Canadian astronomers Bruce Campbell, G. A. H. Walker and Stephenson Yang announced the detection of a planet orbiting Gamma Cephei, an orange dwarf star located about 45 light-years from Earth. However, this discovery was not confirmed until 2003.

On January 9th, 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting PSR 1257+12 - a pulsar located 2,300 light-years away. Follow-up observations confirmed these results and a third planet was confirmed in 1994.

How many exoplanets have we found?

To date, astronomers have confirmed the existence of 4,131 planets beyond our Solar System. Of these, the vast majority have been a combination of Neptune-like gas giants (1,385), Jupiter-like gas giants (1,299), Super-Earths (1,280). Only 161 have been rocky planets that are similar in size to Earth (aka. "Earth-like").

Of all the planets we have discovered, only 55 have been identified as being capable of supporting life - what astronomers refer to as "potentially habitable." Most of these (34) fell into the range of Super-Earths to "mini-Neptunes", 20 were similar to Earth, and 1 was about the same size as Mars.

Not bad considering that all of these discoveries have taken place in just over thirty years. But in truth, most were discovered after 2009 when the Kepler Space Telescope was launched. Since then, a number of missions have built upon this impressive legacy, and more are still to come...

Looking for Life Beyond Earth: The Search for Habitable Exoplanets
Artist's impression of potentially habitable planets, Source: NASA/JPL-Caltech/R. Hurt (SSC-Caltech)

What does "Earth-like" mean?

Simply put, Earth-like planets are those that are believed to be similar in structure and composition to Earth. Earth is composed primarily of silicate minerals and metals that are differentiated between a silicate crust and mantle and a metallic core.

The technical term for this type of planet is "terrestrial", though astronomers often use the term "rocky" to differentiate them from gas giants (which are primarily composed of hydrogen and helium with some heavier elements concentrated in the core).

Beyond structure and composition, "Earth-like" is also meant to imply that a planet has conditions similar to that of Earth. This would include the presence of a thick atmosphere and liquid water on its surface.

What about "potentially habitable"?

This term has also seen a lot of use in recent years whenever the subject of exoplanets comes up. What it refers to are those exoplanets that have been found orbiting within their star's circumstellar habitable zone (HZ), which is sometimes referred to as the "Goldilocks Zone."

RELATED: WHAT IS MEANT BY "HABITABLE ZONE" AND HOW DO WE DEFINE IT? 

This zone corresponds to the distance where a planet orbiting the star will be able to maintain liquid water on its surface. In other words, the planet will have surface temperatures that range from 0 to 100 °C (32 to 212 °F). The range of a star's HZ depends heavily upon the type of star in question.

For example, O, B, A-type stars (aka. "blue giants") have wider habitable zones due to the fact that they are bigger, brighter, and hotter than any other class of star. However, they are also relatively rare, accounting for about 1 in 3,000,000 (O-type), 1 in 800 (B-type), and 1 in 160 (A-type) of the stars in our galaxy.

Looking for Life Beyond Earth: The Search for Habitable Exoplanets
Habitable zones around different types of stars, Source: NASA/Kepler Mission/Dana Berry

F-type stars are those that are blue-white in color and generally only a few times more luminous and massive than our Sun. These stars are more common, making up about 3% (1 in 80) stars in our galaxy.

Then there are G and K-type (yellow and orange dwarf) stars, which make up around 7.5% (1 in 13) and 12% (1 in 8) of the stars within our stellar neighborhood. Our Sun is an example of a G-type star, and these and K-types have relatively tight and narrow habitable zones.

Last, there are the low-mass, cooler, and dimmer stars known as M-type (red dwarfs). These stars are the most common type in the Universe, accounting for about 85% of the stars in our galaxy alone. Typically, they are about 7.5 to 60% the size and mass of our Sun and only 7% as bright. As a result, their habitable zones are rather narrow and very tight.

Okay, now that all of that is covered, let's move on to the matter of how we look for these planets and what we're looking for.

How do we search for exoplanets?

The most popular and effective method for detecting exoplanets is known as the Transit Method (Transit Photometry). This consists of monitoring distant stars for periodic dips in brightness, which could be the result of planets passing in front of the star (aka. transiting) relative to the observer.

This method is very effective at providing information on a planet's size and orbital period (but not it's mass). Not only do the drops in brightness give astronomers a good idea of the planet's diameter, but the timing shows how rapidly it is orbiting its star (and what distance).

Another highly-reliable means of hunting for exoplanets is known as the Radial Velocity Method (Doppler Spectroscopy). This involves observing stars for changes in spectra, which are indications of gravitational interaction between a star and one or more planets (which causes the star to "wobble").

Looking for Life Beyond Earth: The Search for Habitable Exoplanets
Artist's impression of a range of exoplanets that have water, Source: NASA

Basically, when a star is moving away from an observer, its light is shifted towards the red end of the spectrum. When a star is moving away, its light is shifted towards the blue end of the spectrum. This "redshift" and "blueshift" allows astronomers to determine rapidly a star is moving.

This method is very useful for providing estimates on a planet's mass (but not its size or orbit) since the star's "wobble" is directly proportional to the mass of its planetary system.

As Einstein revealed with his General Theory of Relativity, massive objects (like stars, galaxies, and galaxy clusters) warp the fabric of space. This effect causes light to bend and magnify in the presence of a large gravitational field. For decades now, astronomers have used this effect to study distant objects.

When it comes to exoplanets, astronomers use a slight variation of this technique known as Gravitational Microlensing. In this case, the gravity of a star or planet is used to focus and magnify the light of a more distant star, which can make spotting orbiting planets easier.

There's also the direct approach, aka. Direct Imaging, which consists of observing light reflected from exoplanets as they orbit their star. By examining the spectra of this light, astronomers are able to get a good sense of their atmospheres' composition.

Unfortunately, this method is only effective where particularly massive planets (gas giants) that orbit massive stars at great distances are involved. In the case of smaller, rocky planets that orbit more closely to their stars (similar to Earth), the light of the star drowns out anything reflected off their atmospheres.

A number of advances are being made that will allow astronomers to observe smaller planets that have tighter orbits around lower-mass stars. These include observatories with larger mirrors, greater resolution, and adaptive optics, as well as coronographs and spacecraft that can block out the light of a star.

To date, the vast majority of exoplanets discovered have been detected using the Transit Method (76.3%), followed by the Radial Velocity Method (19.2%), the Microlensing Method (2.1%) and Direct Imaging (1.2%), with the remainder having been found using various other methods.

How do we determine habitability?

To be clear, simply knowing if a planet is rocky and whether or not it orbits within a star's HZ does not mean a planet is definitely habitable. Hence why astronomers affix the qualifier "potentially" in front of the world when describing possible candidates.

That being said, the orbit and nature of the planet are good starting points for searching for life "as we know it." Here is another important qualifier. When it comes right down to it, scientists know of only one planet in the Universe that is capable of supporting life (Earth) and the various kinds of life that exist here.

In this respect, exoplanet-hunters are on the lookout for what is known as "biosignatures." These are the telltale indicators of chemicals and elements that are either necessary for life or associated with the existence of past/present life (again, as we know it).

Using Earth as a template, we know that life as we know it depends upon the atmospheric balance of nitrogen gas (N2), oxygen gas (O2), carbon dioxide (CO2) and water vapor (H2O). But of course, Earth has evolved considerably since it formed 4.5 billion years ago, during which time, life has also evolved.

Oxygen gas is a good indicator, since it is not only essential to life on Earth but is also a byproduct of photosynthesis. Speaking of which, carbon dioxide (CO2) is essential for photosynthetic lifeforms (plants and bacteria) and is a greenhouse gas that is effective at stabilizing temperatures.

Then you have ozone (O3), an essential part of the Earth's atmosphere that helps protect life from harmful radiation. There's also methane (CH4), an organic molecule that is the byproduct of anaerobic microbial metabolism (aka. methanogenesis). 

Hydrogen gas (H2) is another indicator since it can act as a greenhouse gas, is a possible indication of volcanic activity and plate tectonics (considered essential to life here on Earth). It is also a byproduct of photolysis, a process that occurs when water is subjected to ultraviolet radiation.

This causes water molecules to break down into hydrogen and oxygen gas. The hydrogen gas escapes to space while the oxygen gas is retained as part of the atmosphere. In other words, the presence of hydrogen gas is an indication of water on a planet's surface.

Other chemicals include nitrous oxide (N2O), methyl chloride (CH3Cl), ammonia (NH3), ethane (C2H6), and various sulfides - all of which are associated with biological processes. Scientists will look for these elements by studying spectra obtained from an exoplanet's atmosphere.

Note the use of the word "will". At present, our instruments are not capable of obtaining spectra from exoplanet atmospheres - at least, not from smaller, rocky ("Earth-like") planets that orbit closely to their stars. But, as mentioned earlier, next-generation telescopes are coming that will change all that.

It's all about the instruments

This includes ground-based and space telescopes that will be launched or begin collecting light within the next ten years. Examples of the former include the Extremely Large Telescope (ELT) that is currently under construction in Chile and will begin collecting light in 2025.

There's also the Thirty-Meter Telescope (TMT), located at Mauna Kea Observatory in Hawaii. Despite ongoing controversy, since the telescope is being built on the sacred ancestral land of the indigenous Hawaiin people, the TMT International Observatory expects operations to commence by 2027.

And there’s the Giant Magellan Telescope (GMT), which is currently being built by the Carnegie Institution for Science (CIS) at the Las Campanas Observatory. Once complete (scheduled for 2025), this observatory will rely on its extreme adaptive optics (GMagAO-X) instrument to directly image exoplanets.

In 2021, the James Webb Space Telescope (JWST), which is the result of extensive international collaboration, will finally launch. This infrared observatory will rely on a 6.5-meter primary mirror composed of 18 ultra-light beryllium segments and a suite of cameras and spectrometers to conduct the most detailed observations to date.

This will be followed by the launch of the ESA's Planetary Transits and Oscillations of Stars (PLATO) in 2026. This telescope, which is part of the agency's Cosmic Vision program, PLATO will attempt to characterize terrestrial planets that orbit within the HZs around Sun-like stars.

And by 2025, NASA will send the Wide-Field Infrared Space Telescope (WFIRST) to space. This observatory will combine a wide field of view with advanced spectrometers and coronographs to conduct observations with the power and precision of about 100 Hubble Space Telescopes.

Where's the best place to look for life?

Now that is a tough question! On one hand, G-type (yellow dwarf) stars seem like a promising target given that our planet orbits a star of this same class. Unfortunately, G-type stars are somewhat rare in our galaxy and only a handful of potentially habitable planets have been discovered around them.

For instance, the closest known-exoplanets that orbit G-type stars are Tau Ceti e, located 12 light-years away; HD 20794 e, located 20 light-years away; Kepler-22b, located 612 light-years away; Kepler-452 b, located 1402 light-years away; and Kepler-1638 b, located 2491 light-years away.

As you can see, these six candidates are dispersed over a rather large area and all of them are Super-Earths that are anywhere from 1.5 to 5 times the size of Earth. Based on official mass estimates, many of these worlds are believed to be covered by very deep oceans (i.e. "water worlds").

Perhaps the most-common M-type red dwarfs then? Of all the terrestrial exoplanets discovered, all that were comparable in size to Earth were found orbiting nearby red dwarfs. This includes the closest exoplanet to our Solar System (Proxima b) and the seven-planet system of TRAPPIST-1.

However, red dwarfs are known for being variable and unstable in terms of the amount of light and radiation they put out. And when they flare, they flare big! In some cases, the flares they emit are powerful enough that they would destroy the atmosphere of any planet orbiting them.

In addition, red dwarfs have tight and narrow habitable zones, which means that any potentially habitable planets would have to be orbiting very close to the star. This would likely result in them being tidally-locked, where one side is constantly facing the star and the other is in perpetual darkness.

Looking for Life Beyond Earth: The Search for Habitable Exoplanets
Artist's impression of Proxima b, Source: ESO

This would mean that one side of the planet would experience intense heating while the other would be freezing cold. At the same time, astronomers have conducted studies and climate simulations that have yielded encouraging results.

For instance, they found that a sufficient amount of water on the planet's surface would generate a dense cloud layer that could shield the surface from much of the incoming radiation. The presence of a thick atmosphere and oceans could also facilitate heat transfer to the dark side.

Beyond the type of star a planet orbits, there's also the degree to which it is similar to Earth. This is known as the Earth Similarity Index (ESI), a concept that was first proposed in a 2011 study by Prof. and an international team of colleagues from the Planetary Habitability Laboratory (PHL), the SETI Institute, and NASA Ames Research Center.

The ESI incorporates the key parameters of a planet (i.e. radius, density, gravity, and surface temperature) into a single numeric value. In their study, Prof. Schulze-Makuch and colleagues indicated that this metric:

"[A]llows worlds to be screened with regard to their similarity to Earth, the only known inhabited planet at this time. The ESI is based on data available or potentially available for most exoplanets such as mass, radius, and temperature."

In the same study, they also proposed a second-tier in the search for life known as the Planetary Habitability Index (PHI), which took into account the "presence of a stable substrate, available energy, appropriate chemistry, and the potential for holding a liquid solvent."

Looking for Life Beyond Earth: The Search for Habitable Exoplanets
Artist's impression of the surface of Gliese 667C c, Source: ESO

In other words, the PHI comes down to geological and surface conditions that current instruments simply can't provide. As such, the PHI must wait for future missions that can provide this kind of detailed information. In the meantime, the ESI remains the only metric that can be used.

Mathematically, the ESI can be expressed as:

http://chart.apis.google.com/chart?cht=tx&chs=1x0&chf=bg,s,FFFFFF00&chco=000000&chl=%7B%5Crm%7BESI%7D%7D%28S%2CR%29%20%3D%201%20-%20%5Csqrt%20%7B%5Cfrac%7B1%7D%7B2%7D%5Cleft%5B%20%7B%7B%7B%5Cleft%28%20%7B%5Cfrac%7B%7BS%20-%20%7BS_%20%5Coplus%20%7D%7D%7D%7B%7BS%20%2B%20%7BS_%20%5Coplus%20%7D%7D%7D%7D%20%5Cright%29%7D%5E2%7D%20%2B%20%7B%7B%5Cleft%28%20%7B%5Cfrac%7B%7BR%20-%20%7BR_%20%5Coplus%20%7D%7D%7D%7B%7BR%20%2B%20%7BR_%20%5Coplus%20%7D%7D%7D%7D%20%5Cright%29%7D%5E2%7D%7D%20%5Cright%5D%7D%20

S is stellar flux, R is radius, S is Earth's solar flux, and R is Earth's radius.

Some promising candidates

In the coming years, next-generation telescopes are going to be aimed at confirmed exoplanets that have deemed worthy of follow-up observations. Using the ESI as a metric, the following exoplanets would seem like a good place to start. Here they are, the top 10 exoplanets to watch for in the next few years:

Teegarden b:

This confirmed exoplanet is the most "Earth-like" planet discovered to date, with an ESI rating of 0.93 (93% similar to Earth). It orbits within the HZ of Teegarden's Star, a red dwarf star that is about 12 light-years from Earth.

The planet is terrestrial and is roughly 1.02 times the size of Earth and 1.05 times its mass. It orbits closely with its star and takes less than five days to orbit its planet (meaning a single year is less than a week here on Earth).

K2-72 e:

This exoplanet, which has an ESI of 0.9 and orbits within the HZ of a red dwarf located about 217 light-years away. It is likely to be rocky and is estimated to be 1.29 times the size of Earth and 2.21 times as massive (putting it in the Super-Earth range). It is also tidally-locked and orbits its star with a period of 24.2 days.

GJ 3323 b:

Also known as Gliese-3323 b, this planet also has an ESI of 0.9 and orbits a red dwarf star 17 light-years away. It too falls in the super-Earth range, with a diameter estimated to be 1.23 times that of Earth and a mass 2.02 times that of Earth. It also orbits closely to its star (0.03282 AU) and completes a single orbit in 5.4 days.

Looking for Life Beyond Earth: The Search for Habitable Exoplanets
Artist's impression of a terrestrial planet orbiting a red dwarf star, Source: ESO/M. Kornmesser

TRAPPIST-1 d:

This planet is one of seven rocky planets that orbit the red dwarf star TRAPPIST-1, located 41 light-years from Earth. It has an ESI of 0.89, is roughly 0.772 times the size of Earth and 0.41 times as massive (making it an example of a subterranean exoplanet). It also has a very tight orbit with its star and takes only 4 days to complete a single orbit.

GJ 1061 c:

Also known as Wolf 1061 c, this planet was called the "nearest potentially habitable planet to Earth" at the time of its discovery (2015). However, scientists have since placed it in the Super-Earth category since it is 1.66 times the size of Earth and 3.41 times as massive.

It has an ESI of 0.88 and orbits a red dwarf star located about 12 light-years from Earth. It has a relatively tight orbit of 0.89 AU and takes 17.9 to complete a single orbit of its star.

TRAPPIST-1 e:

Also located in the TRAPPIST-1 system, this rocky exoplanet has an ESI of 0.87. Like TRAPPIST-1 d, TRAPPIST-1 e is also a relatively diminutive planet, being 0.918 times the size of Earth and 0.62 times as massive. This planet also has a tight orbit and takes just over 6 days to complete a single orbit.

GJ 667 C f:

Also known as Gliese 667 C f, this potentially-rocky planet has an ESI of 0.87 and orbits a star located 22 light-years away. It is 1.45 times the size of Earth, 2.7 times as massive, and has a tight orbit of 0.156 AU, which results in an orbital period of 39 days.

Proxima b:

Located around Proxima Centauri, a red dwarf star that is located just 4.24 light-years away, Proxima b is the closest planet beyond the Solar System. It has an ESI of 0.87, is similar in size and mass to Earth (1.08 times the radius and 1.27 times the mass), and is likely to be tidally-locked to its star - which it orbits with a period of 11.2 days.

Based on recent climate modeling, scientists at the NASA Goddard Space Flight Center determined that Proxima b could be habitable. This is based on the presence of a sizeable ocean and a dense atmosphere, which would allow for heat transfer between hemispheres and radiation protection.

Kepler-442 b:

This rocky exoplanet has an ESI of 0.85 and orbits a K-type (orange dwarf) located 1,115 light-years away. It is roughly 1.34 times the size of Earth, 2.36 times as massive, and orbits its star at a distance of 0.49 AU (half the distance between Earth and the Sun), resulting in an orbital period of 112.34 days.

GJ 273 b:

Coming in at number 10 with an ESI of 0.84 is Gliese 273 b, a rocky planet that orbits a red dwarf located 12 light-years away. This planet is 1.51 times the size of Earth, 2.89 times as massive, and orbits its star with a period of 18.6 days.

‾‾‾‾‾‾‾‾‾‾

It is an exciting time to be alive, thanks to all the groundbreaking work that is taking place in multiple fields of astronomy. And with multiple cutting-edge observatories joining the search in the coming years, the number of confirmed exoplanets is expected to reach into the tens of thousands.

And given the current average (about 1%), tens of thousands of exoplanets will mean hundreds of potentially-habitable candidates. And if only 1% of these have life on them, that's still a handful of planets where alien civilizations could exist!

When that happens, we can expect Frank Drake and Enrico Fermi to be smiling from ear to ear!

Further Reading:

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